Earlier this year, Microsoft officially removed the Books section from the Microsoft Store, taking away users' ability to buy new books and even read existing books after July. This meant that users would need to find a different platform to buy their books from, but at least they could still read them natively in Windows 10 thanks to the Edge browser and its support for the ePub file format.
As it turns out, that's going away too, according to a new support page on Microsoft's website (via Windows Central). The company is removing support for the ePub file format in Microsoft Edge at some point in the future, though there's no clear date for when that will happen. We've reached out to Microsoft for more information on this, but it's likely that support will be dropped when the new Chromium-based version of Edge replaces the current UWP app. If you'd like the capability to return in the new version of Edge, you'll want to make that request to Microsoft through the Tech Community forums for the Edge Insider program.
In the meantime, if you've been using the Edge browser to read books on your PC, there are already some apps in the Microsoft Store that might be able to help. Microsoft says it's worked with the DAISY Consortium to compile a list of apps for reading ePub books, and that those will be available through the Microsoft Store after next month.
Update: Microsoft has confirmed that the end of support for this feature is coming with the release of the Chromium-based version of Edge. Since the UWP version of Edge is still included with Windows 10, you should be able to read ePub e-books in Edge for a while longer.